Gorongosa Field Notes—Jen Guyton: Architects of Lake Urema

In a regular feature on our blog, “Gorongosa Field Notes,” we will be showcasing journal entries, short videos, photographs, and other materials from a team of scientists working at the Gorongosa National Park and the E.O. Wilson Biodiversity Laboratory at Gorongosa National Park in Mozambique. These scientists study the Gorongosa ecosystem—and the critical role of biodiversity—as part of the Gorongosa Restoration Project.

Animal trails cut channels into Lake Urema’s floodplain.

It’s hard to imagine a hippo in hiding. Three-ton packets of honking, huffing, shining grey skin and muscle aren’t easy to conceal, at least not for long. But we couldn’t find them. Here in Gorongosa, these hulking tanks with teeth are shy. Nervous. Like Gorongosa’s elephants, the hippos were persecuted, forced during Mozambique’s long civil war to master the art of camouflage. As we thwocked above Lake Urema in a glistening blue helicopter, only softly undulating ripples tugging at the corner of my eyes hinted at hippos. By the time I turned my attention, there was nothing but the white glare of the sun.

Lake Urema is critical habitat for wetland birds, like this spoonbill and heron.

I’m here to learn more about the park’s wetlands, its coursing rivers and its pulsing nucleus: Lake Urema. And the hippos are the architects of the lake—or at least, they were, before their numbers plummeted from 3,500 to a mere 100 in a quarter-century of illegal hunting during and after the war. Now, the hippo population is recovering. In the twenty years since the war ended, it has more than doubled; there are now about 250 hippos bobbing through the waters of Urema and its tributaries.

In Botswana’s Okavango Delta, hippos forge deep channels, pushing sand aside as they move to and from forage each night. Those channels redirect water, create habitat for aquatic animals, and increase the variety of those landscapes. From a helicopter, such channels are evident in Lake Urema, too. Hippos must have once played a big role in shaping the lake and its floodplain; now, the very small population might be too mobile to have a major impact. Despite being biologically bound to surface water, hippos aren’t great swimmers—their aquatic peregrinations rely on a twinkle-toed dance along the lakebed. So, they follow the shallow water and the good forage, and at such low densities, they aren’t constrained by neighboring territorial pods. It’s critically important to know what’s happening in the lake now that there are so few.

But hippos are never easy study subjects. They’re an ecologist’s nightmare: aquatic, nocturnal, dangerous, with jaws that could chomp a small motorboat in half. Not to mention the fact that they shack up with crocodiles, which recently superseded hippos as the most dangerous animal in Africa.

Launching the boat for a scouting trip onto Lake Urema.

To finally see Gorongosa’s hippos we had to swap the helicopter for a motorboat. We skimmed along the Sungue River, past monitor lizards the size of small crocodiles and crocodiles the size of small cars. Occasionally we caught a glimpse of a humped, hippo-like figure in the distance, where the shining lake seamlessly merged into the white sky. Finally we saw a few brown-gray figures lounging on the bank, unmistakable when one of them stood up to glare at us.

“Those are hippos. Lots of them,” our resident media guru, Bob Poole, observed.

“I can see about ten.” I replied.

“I don’t know… maybe twenty.” He said, handing me his binoculars.

We quieted the engine and cruised closer, and as the hippos staggered from their nap and bombed into the water, we counted. Forty. Forty hippos. That was almost 20% of the park’s entire population in one pod, 100 meters from our boat. They puffed stagnant lake water and stared at us with eyes like river pebbles. Two of the biggest males huffed in our direction and disappeared, folding their ears back and sinking silently. We gunned the boat out of there.

For now I have to leave this place, gather my thoughts, compress and rework my still-buzzing understanding of this quietly complex ecosystem. What we know now about the floodplain and its inhabitants, even the big ones, is just the tip of the iceberg – the South African analog to that expression is “the ears of the hippo”. Scientifically speaking, compared to other charismatic megafauna, the hippopotamus is a blank spot on the map. When it comes to most things hippo, we’ve just barely seen the ears.

—Jen Guyton